Monitoring and Modelling in the VRD

by Alaric Fisher,  Parks & Wildlife Commission of the NT and Adam Liedloff, CSIRO Sustainable Ecosystems

From Managing for healthy country in the VRD, Tropical Savannas CRC 2000.

Modelling the trade-off between fire and grazing | The model | Methods | Results | Discussion | Modelling vegetation change

Satellite information is being used to assist in the monitoring of a number of rangeland sites across the VRD. The attraction of using satellite information is that it enables the extrapolation of land condition and trend information across large areas and reduces the need for intensive ground based sampling.

Click here for a large version of this image

Regional trend product for the Ord-Victoria region from 1992-97 using the average of Landsat MSS band 1 + band 2. The area analysed is 66,550km2 of the 126,000 km2 within four mosaiced Landsat scenes. Light green represents areas where vegetative cover has increased; areas in red a decrease in cover and dark green represents stable cover. In this example, fire scars have not been removed and much of the red is attributed to burnt country. Fire history vectors overlain on this image would aid in identifying areas affected by fire. Clearly these data show that over most of the Ord-Victoria region cover has increased. This trend can be directly attributed to an exceptional run of good seasons from 1993-97.

Work was undertaken to determine whether there is a correlation between land condition assessed with the aid of satellite information, and vertebrate biodiversity. The idea was to determine what species are more likely to be found in sites classified under land condition criteria ranging from "poor" to "good". This would enable much broader scale assessments of vertebrate impacts to be made from satellite information, without the need for labour intensive field based sampling.

While some differences were found in the type of vertebrates found at "good" and "poor" sites, the relationships were quite weak. Unfortunately, the effects of canopy cover and landscape variations complicated assessments of vertebrate impact. Much more work will need to be done to use this technology for assessment of vertebrate impact.

Modelling the trade-off between fire and grazing

Fire and grazing are known to influence the tree-grass character of tropical savannas. Preliminary results from computer modelling has confirmed that savannas in the VRD are reliant on intense fire to maintain their open nature and suppress woody vegetation. Moderate grazing, however, reduces forage and therefore provides insufficient fuel loads to maintain intense fire. A balance therefore needs to be found which takes account of the relationship between fire and grazing intensities.

The model

Computer modelling enables a range of scenarios to be studied that would either be far too costly or not feasible with field experiments alone (See completed CRC project, Modelling and landscape change link below). In this project, the SAVANNA model was used to simulate the structure and function of savannas in the VRD over a 40-year period. The model incorporates factors such as plant population and primary production (the amount of plant biomass such as roots, leaves and stems produced over time) and the role of light, water, weather, fire, soils and grazing pressure. Weather, soils and vegetation information from Kidman Springs was fed into the model and this allowed comparison with results from previous field-based studies.

Methods

Simulations were performed on two 500 metre by 500 metre plots. One was on red calcareous loam soils supporting eucalypt woodlands and the other on black clay soils with a ribbon/blue grass grassland. A range of fire regimes and grazing pressures were simulated to determine the response of the two vegetation types to disturbance. These included mild and intense fires to represent controlled early dry season fires and uncontrolled late dry season fires respectively. These fires also varied in frequency. Grazing was simulated using moderate and heavy grazing pressures.

Results

In the absence of fire, the model revealed a gradual increase in trees and shrubs, particularly on the red soil sites. Tree cover was not influenced by mild fire, but was reduced by intense fire. Shrubs, however, were more susceptible and cover was found to decline with any fire regime. Fire not only reduced woody vegetation cover, but also changed the structure of woody plant populations. The proportion of the smallest trees and shrubs was found to increase as those of intermediate size were killed and replaced by seedlings and re-growth. Fire also led to a decline in perennial grass, which was most noticeable in red soil woodlands.

Like fire, the two levels of grazing simulated also caused a decline in perennial grasses at the red soil site. The more resilient black clay grassland was able to withstand these levels of grazing with only slight declines in yield.

Discussion

These results suggest that intense fire may be an economic means of controlling woody plant populations. Fires will, however, reduce pasture biomass and this must be considered if pasture condition is important for grazing. Moderate to high grazing pressure not only reduces the chance of intense fires, but may also mean that even mild fires used to stimulate grass shoots and improve pasture may not be possible, allowing woody vegetation to increase.

It may not be sufficient to select a simple fire regime in management and expect all facets of the savanna to be maintained. A trade-off between the desirable outcomes of fires, such as reduced woody growth and improved areas of pasture, and the undesirable effects on pasture condition, must be balanced.

Modelling vegetation change

Computer modelling is being used in this project to explore how rangeland vegetation might change over long periods of time under different management and climate change scenarios. The model being developed is designed to overcome some of the limitations of existing models so that more specific and reliable management conclusions can be drawn. One of the key management issues to be studied with the model is the trade-off between grazing and fire in different savanna environments. A wide range of environmental information will be fed into the model and an assessment will be made regarding what scale and level of detail is necessary to provide the best results.

The model will simulate long term vegetation dynamics in terms of the amount and type of dry matter produced. It will explore the role of plant traits such as root: shoot ratios, rooting depth, and associated growth potential, in combination with the distribution of resources in different rangeland environments over space and time.

Feedback mechanisms between vegetation and soils will be factored into the model as well as changes in the composition and physiological characteristics of vegetation. The latter will include changes in the structure of the canopy and root system of plants and the composition of the actual plant tissue.

Changes in vegetation composition will focus on the proportions of annual or perennial grasses and forbs, shrubs and trees, growth and death rates, conditions for the recruitment, establishment, maturing and reproduction of vegetation, and other factors which facilitate the growth potential or competitive ability of pasture plants. This study was part of the now complete CRC research project Modelling Landscape Change .